Hadith: Sulaymān ibn Dawūd (Prophet Solomon) (peace be upon him) said: "I will indeed have sexual intercourse with seventy women at night, and each one of them will give birth to a child who will fight in the cause of Allah."

عن أبي هُريرة رضي الله عنه عن النبيِّ صلى الله عليه وسلم قال: "قال سليمان بن داود -عليهما السلام-: لَأَطُوفنَّ الليلةَ على سَبْعينَ امرأةً، تَلِدُ كلُّ امرأةٍ منهن غُلامًا يقاتلُ في سبيلِ الله، فقِيل له: قل: إن شاءَ الله، فلم يَقُلْ، فطافَ بهنَّ، فلم تَلِدْ منهن إلا امرأةٌ واحدةٌ نصفَ إنسانٍ". قال: فقال رسولُ الله صلى الله عليه وسلم : "لو قالَ: إن شاء الله لم يَحْنَثْ، وكان دَرَكًا لحاجتهِ".
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Sulaymān ibn Dawūd (peace be upon him) said: 'I will indeed have sexual intercourse with seventy women at night, and each one of them will give birth to a child who will fight in the cause of Allah.' It was said to him: 'Say: Allah willing!' But he did not do so. He had intercourse with them all, but none of them gave birth, except for one woman who gave birth to half a person." Upon this, the Messenger (may Allah's peace and blessings be upon him) said: "If he had said, 'Allah willing,' he would not have failed in his oath, and his desire would have been fulfilled."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

Prophet Sulaymān ibn Dawūd (peace be upon him) said to his interlocutor that he would have intercourse with seventy wives of his. It was permissible to combine such a number of wives in Sulaymān's time, or it was something specific to him. His intention was that each woman would give birth to a boy would who fight in the cause of Allah. His interlocutor told him to say: "Allah willing!" But he forgot to do so. He had intercourse with his wives, but none of them gave birth, except for one woman who gave birth to a stillborn. The Prophet (may Allah's peace and blessings be upon him) said that if Sulaymān had said "Allah willing", he would have fulfilled his oath and realized his wish.